I use Dazian grey rear-projection material. I have a large front projection screen (8X16), but the rear-projection screen is much smaller -- only about (5X6), so the cost was reasonable.http://www.dazian.com/cgi-bin/page.p...=165&cat_id=50
I have a colonnade directly in front of our gardenhouse. It has columns that are 6 feet apart. I bought a 7-foot roll up window shade, pulled off the shade material, and replaced it with the Dazian material. I mounted the brackets on the colonnade, so that all I have to do is hang the shade and pull it down. The projector is in the gardenhouse so I just shoot the image out the door onto the screen between the columns. Setup and takedown are so easy that we use this screen more than the large screen.
The image is spectacular!
If I was looking for an inexpensive alternative to the Dazian material I'd look for an opaque, grey vinyl. I was surprised that I couldn't see through the material. The closest alternative I've seen was a ground cloth that goes under a tent. It looked like the same material, but a 15X15 sheet was only $15. I never got to try it so I don't know if it works as well as the Dazian.